The number of birds of prey found dead within a two-mile area has now increased to 19, with a reward for information of around £26,000 being offered.
Tests carried out on the 14 red kites and five buzzards discovered near Conon Bridge in the Highlands have revealed 12 were poisoned.
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As the confirmed death toll rose, it emerged contributions to an online donation page had added more than £4,000 to the total reward sum.
RSPB Scotland initially put up a £5,000 reward, which was quickly doubled by an anonymous contribution. The charity then created a Justgiving site after being inundated with calls from members of the public, with more than 160 people donating so far.
A pledge of £12,000 was also made today by a group of 15 farmers from the Moray Firth area.
The group, comprising members of Scottish Land and Estates and the National Farmers Union (NFU) Scotland, has condemned the deaths and offered to help police in the investigation.
One of the landowners involved in collating the reward is Alex Matheson from the Brahan Estate, which operates the Tollie Red Kites centre in partnership with the RSPB.
He said: "We have been appalled by these tragic and senseless bird deaths. By pulling together as a community to do all we can to help, we want to send a strong and clear message that any form of wildlife crime is totally unacceptable. We would urge anyone who has any information to come forward."
The raptors were found to the south east of Conon Bridge around Conon Brae, Balvail, Leanaig and Alcaig, with tests revealing nine red kites and three buzzards digested a poisonous substance. Post-mortem examinations and toxicology work continues into all the birds, police said.
Duncan Orr-Ewing, head of species and land management for RSPB Scotland, said: "We welcome the pledge made today by local farmers and landowners. The illegal killing of these birds has disgusted many in the local community and beyond and we hope that by working together, we can identify those responsible and help to eliminate wildlife crime.
"We would also like to thank everyone who has given so generously on our Justgiving site in an effort to help bring those responsible to justice. The public outrage over the deaths of these birds is clearly reflected in the comments posted by those donating. Many more will share their anger and frustration and back the repeated calls for tough penalties against the perpetrators.
"We again urge anyone who has any information to contact Police Scotland as soon as possible."
Officers are continuing to appeal for information and advised anyone who finds any further dead bird of prey to note its location and inform them, without attempting to recover it.